1. Field of the Invention
The present invention relates generally to electric power steering apparatuses for automotive vehicles which provide a steering assist of an electric motor directly to the vehicle steering system to reduce steering force to be applied manually by a vehicle driver, and more particularly to an improved electric power steering apparatus which, only when the vehicle is in a substantially stationary condition, permits switching between predetermined characteristics of a plurality of steering assist modes variable as desired by the vehicle driver.
2. Description of the Related Art
In recent years, there has been an increasing demand that an electric steering assist to vehicle driver's manual steering operation be provided with different characteristics depending on the sex, age, driving experience, etc. of the vehicle driver. To address to such a demand, an improved electric power steering apparatus has been proposed in Japanese Patent Laid-open Publication No. SHO-59-100059 which is designed to vary the steering characteristic by switching between normal and sports modes as desired by the vehicle driver. The proposed electric power steering apparatus includes a torque detector for detecting steering torque, a control unit for variably controlling power output from the electric motor in accordance with an output signal from the torque detector, and a servo-rate setting device, positioned within the reach of the vehicle driver operating the automotive vehicle, for setting a desired ratio of the output torque from the motor to a level of input signal to the control. With this electric power steering apparatus, a steering characteristic as desired by the vehicle driver can be obtained by the vehicle driver selecting a servo rate to be set by the servo-rate setting device.
Specifically, the servo rate setting device in the proposed electric power steering apparatus is arranged to vary the relation between the level of input signal to the control and the output torque from the motor and thereby vary the steering characteristic, by setting an amplification factor of an amplifier or an attenuation factor of an attenuator connected between it's torque signal input and the output of the torque detector. The servo rate can be set even during running or travel of the automotive vehicle.
However, in the electric power steering apparatus proposed in Japanese Patent Laid-open Publication No. SHO-59-100059, the output torque from the motor would undesirably vary depending on the servo rate even when the steering wheel is operated with same steering torque, although the steering characteristic can be varied as desired by the vehicle driver even during travel of the automotive vehicle. Because the steering characteristic varies as the servo rate is changed during steering operation, there could occur a sense of "incongruity" in the driver's steering, which would lead to a deterioration in the driver's steering feel. Particularly, variations in the steering characteristic during a high-speed drive of the automotive vehicle would significantly deteriorate the driver's steering feel and also adversely affect the vehicle behavior. Switching between the steering characteristics when the vehicle velocity is sufficiently low (e.g., zero) is most suitable in that it does not adversely affect the vehicle behavior, but the velocity sensor may sometimes malfunction to erroneously detect a zero vehicle velocity although the vehicle is actually running.